STOFFEL M. and BENISTON M. On the incidence of debris flows from the early Little Ice Age to a future greenhouse climate: A cas study from the Swiss Alps. Geophysical Research Letters, 2006, vol. 33.

Laboratory of Dendrogeomorphology / Institute of Geography, Department of Geosciences, University of Fribourg
martin.beniston@unige.ch

Tree-ring based reconstructions of 123 debris-flow events since AD 1570 show that relatively few debris flows occurred during the period from 1570 to 1860. It was due to cooler summers and more frequent summer snowfalls in the debris-flow starting zone. Enhanced activity was observed during the wet periods (1864–1895) following the last LIA glacier advance and in the early decades of the 20th century. In contrast, comparably low activity can be observed since 1995, with only one event recorded.

Decadal frequencies generally remained well below average during most of the Little Ice Age (LIA) and periods with considerable above-average debris-flow activity only start to emerge from the data in the 1860s. Largely increased activity continued well into the early 20th century and culminated in two 10-yr periods between 1916 and 1935, when seven events each were derived from the tree-ring series. It reflects the warm-wet conditions prevailing in the Swiss Alps during that particular period of the last century. This episode of important activity was followed by a rather sharp decrease in the 10-yr frequencies. Very low activity can be observed for the last 10-yr segment (1996–2005) with only one debris-flow event recorded. Along with the periods of 1706–1715 and 1796–1805, the most recent ten years exhibit the lowest debris-flow activity in the last 300 yrs.

The reconstructed frequency is in agreement with chronicle data on extreme flooding events in Alpine rivers of Switzerland, where a scarcity of flooding events can be observed for most of the LIA and the mid 20th century as well. In contrast, floods in Alpine rivers started to become more frequent around the 1830s, i.e., almost three decades before activity increased in the investigated Ritigraben case-study area.

Generally, debris flows occurred much earlier in the summer prior to 1900. This is especially true for the period 1850–1899, when more than 70% of the reconstructed debris-flow events took place in June and July and no incidence occurred in September. In the 20th century, debris-flow activity clearly shifted toward August and September, with not a single event registered for June after 1962. Finally, snowfalls and frozen ground inhibit debris entrainment from the starting zone (>2,600 m) from October to May.

Based on observations of flooding in adjacent rivers, it also appears that debris flows at the case-study site would have been triggered more frequently by spatially limited summer thunderstorms from the 1860s until the 1980s. Since 1987, events have been released by synoptic weather systems located south of the Alps in late summer and fall. The reconstructed shift of debris-flow activity from June and July to August and September can be further explained by the negative trend observed for heavy summer rainfall and the slightly positive trend found in heavy fall precipitation intensities in the study region over the 20th century [Schmidli and Frei, 2005].

While debris flows of the past have occurred mostly during summers with greater total precipitation than average, it is conceivable that in a greenhouse climate the frequency of such events could decrease because of shifts in the occurrence of extreme precipitation from summer to spring or fall by 2100, as suggested by a number of RCMs.

The impacts of future precipitation events may be lower than today, because spring and fall temperatures are suggested to remain 4–7°C degrees below current summer temperatures in a greenhouse climate [Beniston, 2006]; the lower freezing levels that are expected in future springs and falls compared to current summers, and the buffering effects of snow might probably reduce the risk of debris flows to be released from the starting zone.

However, there could be a risk of enhanced runoff in spring if abundant rain falls upon the snow pack. Given that sediment remains readily available in the upper basin and that the channel is regularly recharged with debris, the magnitude and impacts of future summertime debris flows could be greater than currently because of warmer temperatures and higher precipitation intensities, even if the frequency of summer events is likely to decrease.

Temperature and precipitaton

This paper aims to assess the debris-flow activity in a catchment of the Swiss Alps during the period 1570– 1900 (the classic LIA), to document its evolution in the 20th century and to give an insight as to possible changes in the 21st century.

The area chosen for the analysis is the Ritigraben torrent (Valais). In the source area of the torrent (2,600–3,214 m a.s.l.), contemporary permafrost was prospected with geophysical investigations. On its downward course to the Mattervispa river, the torrent passes a forested cone on a structural terrace (1,500–1,800 m a.s.l.), where debris-flow material affects trees.

The investigation was therefore based on 1102 Larix decidua, Picea abies and Pinus cembra trees (2246 cores) that have obviously been disturbed by events in the past. At least two cores per tree were extracted using increment borers, one in the flow direction of past debris flows and another on the opposite side of the trunk. In addition to the affected trees sampled on the cone, we selected 102 undisturbed reference trees from stands located next to the cone so as to separate insect infestations or climatically driven fluctuations in tree growth from disturbances caused by debris flows.

The identification of past events was based on the number of samples simultaneously showing a growth disturbance as well as on their spatial distribution. As conifers react immediately to damage with the formation of TRD, the intra-annual position of the disturbance was further used to assess the moment of debris-flow activity in particular years with monthly precision.

In total, the analysis of signatures occurring simultaneously in different trees on the cone allowed the reconstruction of 123 debris-flow events covering the last 440 yrs.